In the field of information processing, magnetic recording mediums enjoy a brisk demand due to their effectiveness as the element for recording and reproduction. At the same time, there have been increasing demands for improved electromagnetic conversion characteristics, a larger recording capacity and a higher density, improved running properties, durability and storageability, better cost performance, as well as simplified manufacturing procedures. Efforts are being made in the art to satisfy these requirements.
It is generally known that electromagnetic conversion properties can be improved by reducing the thickness of a magnetic layer, increasing the packing ratio of magnetic powder and making the surface of a component layer provided on the side of a magnetic layer smooth. For these purposes, selection of magnetic powders and fillers with suitable particle sizes, as well as calendering treatment are required to be performed.
However, excessive surface smoothness results in an increased friction coefficient and poor running properties. On the other hand, too much roughness causes a spacing loss, powder drop-off, and wear of a magnetic head. From the above, it is readily understood that the surface quality of a magnetic layer is a matter requiring a thorough examination.
It should be noted that the surface of a non-magnetic support provided in a conventional magnetic recording medium (hereinafter, simply referred to as a "magnetic tape") is rough enough to give the surface of a magnetic tape such unevenness as will affect adversely electromagnetic conversion characteristics. Such surface roughness, coupled with the single-layer structure employed in a lot of magnetic layers, produces various unpreferable noises, which cannot be avoided by the provision of a subbing layer.
Taking the case of a video tape, its electromagnetic conversion characteristics, in particular, chroma S/N and lumi S/N, are greatly influenced by its surface quality. Excessive smoothness results in poor running properties, increased rubbing noises, irregular electromagnetic conversion and cinching due to contamination of a magnetic head.
In a multi-layered magnetic layer which is advantageous for both high and low frequency recordings, to minimize the negative effects of the surface roughness of a support, attempts were made to increase the thickness of an upper magnetic layer or to increase the smoothness of a lower magnetic layer. The former method, however, impairs low frequency characteristics imparted to the lower magnetic layer. In the latter method, excessive smoothness of the lower magnetic layer results in increased rubbing noises.
Noises, one of factors constituting electromagnetic conversion characteristics, are generated by problems in the structure and condition of a magnetic tape. The example of noises include drop-out due to defective structure of a magnetic tape or accumulation of dust; electric discharge noises or rubbing noises generated by an electric discharge in the proximity of a magnetic head; and other noises of unknown origin. Among these noises, rubbing noises, reduction of which is not an easy task, have their sources in a magnetic head. A magnetic head, in turn, is greatly affected by the tribology of a magnetic tape.
Factors contributing to an uneven magnetic layer surface include the roughness of the surface of a support, agglomeration of magnetic particles, the in-plane density distribution, and ununiform coating. These factors, coupled with such factor as surface friction coefficient, cause such problems as poor contact between a head and a tape; vibration of a tape due to its expansion and contraction in the longitudinal direction, which is ascribable to poor running performance; and flattering. These problems ultimately lead to generation of rubbing noises.
To avoid them, it is important to make the properties of a magnetic tape, such as electromagnetic conversion properties and running properties, well-balanced.
However, with the recent increase of recording density, it has come to more difficult than ever to attain such balance. Under such circumstances, it is not proper to depend on conventional methods in evaluating the properties of a magnetic tape.
For instance, the center line average roughness (Ra), which has been the most common index for surface roughness, is not suitable for the evaluation of spacing loss. Ra is the average value of definite integrals of the absolute values of the roughness curve f(X) within a reference length (1), which is defined by the formula: ##EQU1##
Ra takes into account concave portions which do not essentially affect the running properties of a tape. In this respect, measurement of Ra for the evaluation of spacing loss may be misleading. Instead of Ra, Rmax, the arithmetic average of heights of projections within a reference length, should be employed for the evaluation of spacing loss. In Rmax, a height includes the length of the lower half of a projection, which is seemed to exert no influence on tape characteristics. According to the above definitions, Ra and Rmax have totally different physical meanings.
The Japanese Patent Publication Open to Public Inspection Nos. 168124/1986 and 143523/1981 describe the influence of Ra and Rmax on electromagnetic conversion characteristics and running properties.